1. Field of the Invention
The present invention generally relates to an integrated circuit having an ultra-low value current source, and more particularly to an integrated circuit incorporating a circuit arrangement for processing sampled analog electrical signals including a feedback apparatus providing an ultra-low value current source (e.g., current reference).
2. Description of the Related Art
Many applications require a plurality of current sources in the microamp to nanoamp region.
For example, portable battery-powered equipment including any of input pin tie-up or tie-down (resistors), key stroke pull-up resistors, current sources, and/or high impedance analog functions (e.g., operational amplifiers, comparators, terminal sensors, etc.) represent applications requiring current sources in the microamp to nanoamp region.
Moreover, stand-by chip power dissipation of less than a microwatt is frequently desired, thereby to reduce power consumption and attendant battery drain. Therefore, individual current sources must be in the nanowatt region. As discussed below, conventional approaches have been impractical or self-defeating.
FIGS. 1A and 1B illustrate first and second conventional examples.
FIG. 1A illustrates a single key pad (e.g., switch) 1 with a pull-up current source. A very high impedance resistor 2 (e.g., having a value of about 100 megohms) forms the current source. A value of a wire and nodal capacitance 3, upstream from an amplifier 4, is on the order of about 10 picofarads (Pfd). The time constant of the node (e.g., approximately 1 millisecond) is adequate for key stroke entry. However, power dissipation (100 nanowatts/ckt.) is undesirably high, and the area required for the resistor 2 is excessive, thereby requiring much chip "real estate" and making the circuit unduly large.
FIG. 1B illustrates a very low current source derived from a much larger current source.
Specifically, a ratioed current mirror is used for providing a smaller current source. A single ratioed current mirror or a cascade of current mirrors 10, 10' and 11, 11' divide and subdivide a well-controlled input current (Vdd)down to an acceptable value (e.g., 10 nanoamps, but of course such a value depends on the application involved). Both transistors of a current mirror pair share the same gate to source potential. Therefore, the currents through the two transistors assume a ratio proportional to their respective W/L ratios.
A resistor 15 is provided as shown to establish the current which is then subdivided. The resistive values of the resistor 15 are reasonably low (e.g., 1 Megohm), and the output current is well-controlled. However, the excessive power consumption of the input reference current overwhelms and defeats the purpose of the low-valued output currents where low power is the primary goal.
Thus, conventional solutions result in either excessive area or excessive and disproportionately higher power, or both.